Catalytic dehydrogenation of hydrocarbons is a well known and commercially important process. The reaction is strongly endothermic. At adiabatic conditions this will result in a lowering of the temperature in the reaction mixture and a consequent lowering of the reaction velocity. Therefore, existing catalytic dehydrogenation processes are dependent on external heat supply to uphold the reaction temperature. Besides, dehydrogenation reactions are subject to equilibrium limitations at typical process conditions.
The above-mentioned limitations have led to the development of autothermal dehydrogenation processes wherein the dehydrogenation is effected in combination with an oxidation of formed hydrogen to water with an oxygen-containing gas. At typical reaction conditions, the exothermic heat generated by the combustion of about half of the formed hydrogen will compensate for the heat loss resulting from the endothermic dehydrogenation reaction. In addition to achieving a desired heat balance, the consumption of hydrogen in the combustion reaction will shift the equilibrium of the desired dehydrogenation reaction in the direction of a higher conversion to dehydrogenated hydrocarbons.
It is strongly desired in such an autothermal dehydrogenation of hydrocarbons that a selective oxidation of hydrogen takes place without any substantial oxidation of hydrocarbons to carbon oxides, as oxidation of hydrocarbons will reduce the efficiency of the dehydrogenation process, due to a direct loss of product and also due to the formation of carbon monoxide. Carbon monoxide may have a deleterious effect on the performance of the dehydrogenation catalyst and thus indirectly reduce the yield of the desired dehydrogenated hydrocarbon.
U.S. Pat. No. 4,914,249 describes a process for autothermal dehydrogenation of a dehydrogenatable hydrocarbon, comprising two dehydrogenation stages and an intermediate oxidation stage for a selective oxidition of hydrogen to water. In this previously known process, the effluent stream from the first dehydrogenation stage, comprising a mixture of dehydrogenated hydrocarbon, unconverted hydrocarbon, hydrogen and steam, is subjected to a selective oxidation of hydrogen on a separate oxidation catalyst in a separate oxidation zone, to which zone the oxygen-containing gas required for the combustion is fed, preferably at a position adjacent to the bed of oxidation catalyst. The effluent from this separate oxidation zone is then subjected to the next dehydrogenation step. Said patent does not specify the flow conditions at which the oxygen-containing gas is introduced into the oxidation zone and mixed with the effluent stream from the first dehydrogenation stage.
U.S. Pat. No. 4,739,124 discloses another autothermal dehydrogenation process. Here, a hydrocarbon represented by ethane is dehydrogenated catalytically in a reactor comprising at least two separate beds of a dehydrogenation catalyst. A feed stream comprising ethane is passed into the first bed of dehydrogenation catalyst maintained at dehydrogenation conditions comprising temperatures in the range of 538.degree. C. to 750.degree. C. The effluent stream from this bed, comprising ethane, ethylene formed as a product and hydrogen formed as a by-product, is cooled and then mixed with an oxygen-containing gas, whereupon the obtained mixture is fed to a separate bed of a selective hydrogen oxidation catalyst maintained at oxidation promoting conditions. The effluent stream from said oxidation bed, which has been heated as a result of the hydrogen combustion, is passed to a second bed of dehydrogenation catalyst similar to the first bed of dehydrogenation catalyst.
The purpose of cooling the effluent stream from the first bed of dehydrogenation catalyst in the process of U.S. Pat. No. 4,739,124, by direct or indirect heat exchange, is to increase the need for combustion of hydrogen in the bed of hydrogen oxidation catalyst. Because a larger part of the hydrogen in the gas mixture has to now be consumed to reach the desired dehydrogenation temperature, the equilibrium concentration of dehydrogenated hydrocarbon i the gas mixture is increased, and the higher equilibrium concentration becomes a driving force for achieving an increased conversion in the dehydrogenation reaction.
The distribution and admixing of the oxygen-containing gas in the process of U.S. Pat. No. 4,739,124 takes place in a catalyst-free zone between the first bed of dehydrogenation catalyst and the bed of hydrogen oxidation catalyst. The oxygen-containing gas may be introduced co-currently through nozzles (shown in the drawing of the patent) or, as also suggested in the patent, through a complex grid of piping having a circular or branching structure (column 8, lines 54 to 58). It is also mentioned that various elements may be placed in the catalyst-free zone to improve the mixing efficiency, but the utilization of such elements is not recommended as it would tend to increase the reactor costs and may increase the pressure drop through the process. There is no suggestion in the patent that the catalyst-free zone should have a restricted flow cross-section area relatively to that of the two beds of dehydrogenation catalyst, for the purpose of increasing the flow velocity in the catalyst-free oxygen-admixing zone.